The present invention relates to bioelectrical measurement, particularly to a method for localizing electrical activity in the body, such as in the brain or heart.
As is well known, the body produces electrical fields and waves, and these provide important information regarding body function. To acquire this information, it is often necessary and usually desirable to determine the sources of electrical energy inside the body whose electrical effects are measured at the body surface. Conventional source analysis begins by measuring the electrical potential at the surface of the body proximate an underlying organ of interest. For analyzing sources in the heart, an electrocardiograph ("ECG") measures the electrical potential on the torso, while for analyzing sources in the brain, an electroencephalograph ("EEG") measures the electrical potential on the head.
To localize the sources in the organ or tissue that are responsible for given measured surface potentials, a computer-based model is made of the organ or tissue in terms of the conductive paths leading therefrom to the body surface. Ideal sources, such as single or multiple dipoles or extended dipolar sheets, are modeled in the computer and manipulated within a model of the organ or tissue while surface potentials are calculated until satisfactory agreement is reached between the calculated values of potential and those actually measured on the body. Such models require specification of both the body geometry and the body impedance or conductivity as a function of position within the body.
In conventional EEG models, the head is represented by a small number of spherically concentric tissue layers or regions, typically divided as brain, cerebrospinal fluid, skull and scalp, simplifying the model so as to require only four impedance values, one value for each region. In conventional ECG models, the torso is similarly considered to consist of the heart, lung, body cavity and skeletal muscle. Such models make use of impedance values that have been estimated by measuring laboratory subjects.
The advent of magnetic resonance imaging techniques now provides for more geometrically detailed head and torso models, and consequently more precise source localization should be possible. Accordingly, the art has begun to focus on obtaining more precise measurements of impedance of bodily tissue for use in the published tables. However, the increased precision has not translated to the expected degree of increased accuracy. The reason for this has not heretofore been understood.
Accordingly, there is a need for a method for localizing electrical activity in the body that provides for further reducing source localization errors.